SummaryThe main points in this study of hydatid cysts of the peritoneal cavity are:1The cysts, usually multiple, but sometimes single, are with rare exceptions secondary and brought about by leakage from a primary cyst of the liver many years previously. Such leakage more often than not has been insidious or completely forgotten by the patient. Cases are frequently met with; 228 are noted among 1,455 hydatid records in the Hydatid Registry at the end of the year 1939.2Errors in diagnosis are common.3Prognosis is not so bad as it is painted. The prospects of their reaching old age are remote, but most patients have a fairly long and useful life.4Surgical treatment is sooner or later called for and operations may have to be repeated again and again.